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St. Augustine was obsessed with squaring what seemed a circle: How could one be a follower of Christ, whose life exemplified pacifism, yet also justify the use of armed force and, indeed, mass killing, particularly against heathen barbarians bent on ruining Rome and conquering Christianity? Augustine’s answer: A ruler owes his people the duty of protection, thus wars of self-defense are morally permissible. But a ruler’s intent in ordering such wars should only be love for his people and have nothing to do with such deadly sins as hatred, anger, bloodlust or greed. His concept is known as the just war theory.

Millennia in the making, the doctrine continues to influence military policy and political practice worldwide. Its basic tenet is that warfare is sometimes morally permissible. This sets the theory apart from pacifism, which denies this principle, as well as from realism, which stipulates that war and morality have nothing to do with each other and that war ought to be considered solely as a selfish calculus of national interests in things like power, security and natural resources.

Augustine (354–430) is often identified as the doctrine’s founder, though this ignores substantial prior Greco-Roman contributions from the likes of Aristotle (384–322 BC) and Cicero (106–43 BC). The theory’s real-world influence tends to wax and wane in opposition to realism, its main rival. Just war theory has also influenced development of the international laws of armed conflict, for instance through the seminal Dutch lawyer Hugo Grotius (1583–1645) and in the Hague and Geneva Conventions.

Today the doctrine frames basic rules to aid decision-makers confronted by what Tolstoy simply, yet monumentally, referred to as “War and Peace.” These rules fall into three categories, still referred to in their original Latin: jus ad bellum (“justice of war,” regarding political rules for starting wars); jus in bello (“justice in war,” regarding rules for soldierly conduct during war); and jus post bellum (“justice after war,” regarding rules to guide the transition from conflict back to peace).

Jus ad bellum focuses on when, if ever, it is justifiable to unleash what Shakespeare referred to as “the dogs of war.” The consensus is that wars of self-defense from aggression are morally justified. Such wars aren’t simply smart, or “wars of necessity,” they are fully good and morally just. They are so because aggression involves the use of armed force to violate the fundamental rights of individuals and communities to live in peace and pursue happiness as they see fit. To violate such rights is to negate the basic possibilities of human civilization.

Just war theorists have been unable to reach consensus on whether nondefensive wars, or “wars of choice,” can ever be justified. Some, such as Aristotle and Cicero, justified wars of preemption, anticipation and even of empire-building, sporting, shall we say, a robust concept of defense. Others, like the Conquest-era Spaniard Francisco de Vitoria (1492–1546), argued that offensive wars are akin to “punishing a man for a sin he has yet to commit.”

The rules of jus ad bellum permit war only if one can demonstrate: a just cause such as defense of the people; a good intention absent ulterior motives like ethnic hatred or greed; probability of success; and proportionality (the problem is so severe—like fanatical, aggressive conquest—that it truly requires war to check it). Furthermore, one must only resort to war after exhausting peaceful means of dispute resolution, and then publicly declare war through a proper authority.

Jus ad bellum is meant to guide those with the power to make war, often heads of state. Jus in bello, by contrast, is to guide officers and soldiers and includes the following guidelines: Do not directly and intentionally attack civilians; extend benevolent quarantine to surrendered or captured enemy soldiers; observe proportionality between means and ends in planning attacks; do not use prohibited weapons such as chemical or biological arms; avoid punitive reprisals against enemy violations of the laws of war; and shun means “evil in themselves,” such as forcing captured soldiers to fight against their own side. These rules have the deepest pedigree in the theory, predating aristocratic medieval sporting tournaments like the joust all the way back to the Old Testament, which urged that targeting food supplies (like fruit trees) be considered an illegitimate act of war.

Contemporary instances of jus in bello concern the treatment of captured enemy prisoners and terrorists, as well as the status of civilian populations. Just war theory urges against the use of torture and strongly in favor of a robust hands-off policy regarding civilians. Some realists view the mandates as hopelessly quaint, arguing that 9/11 ushered in a new era in which the bare-knuckled struggle for survival should be given free rein. TV shows like 24 encourage such thinking. Many such declarations in the past, however, have proven historically hasty and false. Just war theory counsels that, ultimately, winning well is the best revenge. Michael Walzer, arguably the dean of living just war theorists, explores the notion thoroughly in his book Just and Unjust Wars (first published in 1977, now in its fourth edition).

Jus post bellum is a nascent, cutting-edge aspect of just war theory that addresses such concerns as military disarmament, apologies, war crimes trials, compensation and punishment, publicly declared peace treaties, and aid and rehabilitation. One can witness tension between those preferring more limited postwar ideals, stressing punishment and trials (a revenge paradigm), and those supporting more bold experimentation in institutional change (a rehabilitation paradigm). Successful instances of modern postwar reconstruction—notably the South after the Civil War and West Germany and Japan following World War II (note U.S. presence in all three)—provide rich material of manifest relevance to current world conflicts. Statesman James Dobbins addresses the latter in recent RAND reports. How can, and should, a victor transform the loser’s regime and society in the wake of conflict? What does history say about best practices in this regard, and are similar conditions in place in the current situations?

Just war theory has its critics. Some suggest that linking warfare with justice only makes war more destructive and “crusadelike” in character. Proponents reply that something as serious as war requires moral restraint and justification and that an amoral, cynical approach to war is hardly likely to result in less destruction.


Originally published in the October 2007 issue of Military History. To subscribe, click here